1. Field of the Invention
This invention relates to systems and methods that perform evaluation on electric characteristics of printed-circuit boards in accordance with numerical analysis. In addition, this invention also relates to storage media storing programs and data that actualize evaluation of the electric characteristics of the printed-circuit boards.
This application is based on Patent Application No. Hei 11-114200 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
Conventionally, engineers propose a variety of techniques that perform evaluation using numerical analysis as to whether printed-circuit boards operate normally or not. One of those techniques generally known corresponds to a circuit simulator called “PSPICE” (where “SPICE” is an abbreviation for “Simulation Program with Integrated Circuit Emphasis”, which is developed by the University of California at Berkeley), an example of which is described on pp. 25–27 of RF Design published on January of 1993. Another example of the circuit simulator is disclosed by Japanese Unexamined Patent Publication No. Hei 9-274623, which aims at improvement of accuracy and reduction of number of steps for simulation of transmission lines in upstream stages of designs of electric circuits fabricated on boards.
The content of the aforementioned publication will be described in detail with reference to FIG. 23, which is a block diagram showing a configuration of a transmission-line simulator.
The transmission-line simulator of FIG. 23 operates as follows:
At first, there are provided components of electric circuits, which are being symbolized. The symbolized components (or component symbols) and their connection (or wiring pattern) are input to a display controller 101, so that a display 102 shows on a screen a physical shape and wiring topology with regard to a board on which the components are interconnected together in accordance with the connection. The display controller 101 urges a human operator to select an appropriate property for the connection by means of an input device 103. The property is forwarded to an electromagnetic field simulator 106, which calculates line constants of the connection. Through calculations, the electromagnetic field simulator 106 creates a line model. A substitution block 105 receives the component symbols so as to extract a corresponding device model from a component library 105a. A combination block 107 combines the line model and device model together to form an equivalent circuit, which is a subject being evaluated. Thus, a circuit simulator 108 performs transmission-line analysis on the equivalent circuit with respect to its transmission-line delay characteristic and transmission-line reflection characteristic.
FIG. 24 shows an example of a circuit model in which a driver IC 201 and a receiver IC 202 are mounted (or fabricated) on a printed-circuit board 200. The aforementioned simulator is capable of evaluating electric characteristics of signals, which are transmitted from the driver IC 201 to the receiver IC 202 by way of a line 203.
That is, it is possible to produce an analysis model and its analysis result with respect to the circuit model formed on the board 200 shown in FIG. 24. Namely, FIG. 25A shows an example of the analysis model, and FIG. 25B shows an example of the analysis result.
The analysis model of FIG. 25A simply shows the circuit model in which the driver IC 201 and the receiver IC 202 are connected together by way of the line 203. Herein, an equivalent circuit being formed using a voltage source (or current source) and a certain value of impedance is substituted for the driver IC 201, while an equivalent circuit being formed using a certain value of impedance is substituted for the receiver IC 202. In addition, the line 203 is replaced with a simple equipotential line or a transmission line which is constructed by combining resistors, inductors and capacitors, for example.
The analysis results of FIG. 25B show a voltage waveshape, which is being measured at an input of the receiver IC 202. This voltage waveshape includes relatively large “overshoot” in a rise portion, and it also includes “undershoot” in a decay portion. Such voltage waveshape cannot guarantee a normal operation of a subject circuit which is a subject being evaluated. Hence, it can be said that a printed-circuit board having such a subject circuit is inappropriate for manufacturing.
To improve electric characteristics, it is possible to propose another analysis model shown in FIG. 26A, in which a filter circuit 204 is inserted between the driver IC 201 and the line 203. By insertion of such a filter circuit 204, it is possible to suppress the overshoot and undershoot in the voltage waveshape, which is shown in FIG. 26B.
Using the aforementioned operations of the transmission-line simulator, it is possible to change circuit parameters with ease. Thus, it is possible to optimize circuit design with ease. In addition, it is possible for engineers to grasp a signal waveshape of the circuit before actual manufacture of the printed-circuit board having the circuit. This reduces reprints of printed-circuit boards due to errors in circuit design. So, it is possible to reduce an amount of cost in manufacture of the printed-circuit boards.
In some cases, the printed-circuit boards are designed to have power supply circuits, which supply active components such as ICs and LSI devices with stable direct-current voltages, other than the aforementioned circuits that perform transmission and reception of signals. FIG. 27 shows an example of an equivalent circuit which is created in connection with the power supply circuit. That is, the equivalent circuit of FIG. 27 contains a power supply circuit 210, which corresponds to an area encompassed by a dotted line.
Specifically, the equivalent circuit of FIG. 27 contains an IC 205 being connected with a power terminal 206 and a ground terminal 207, between which a capacitor 208 and a DC power source 209 are connected. Herein, the capacitor 208 acts as the power supply circuit 210, while the DC power source 209 is provided outside of the board. In addition, the capacitor 208 is connected in proximity to the IC 205. This provides replacement of the DC power source 209. That is, the capacitor 208 supplies the IC 205 with electric charges, which are required for the IC 205 to perform switching operations. Incidentally, it is possible to use a low-impedance capacitor as the capacitor 208. In that case, no variations are caused to occur in voltage between the power terminal 206 and ground terminal 207 even when the IC 205 performs the switching operations.
The aforementioned capacitor 208 is called a decoupling capacitor. Until recently, it is believed that the conventional circuit simulators do not have to analyze high frequency characteristics of the power source circuits, which are assumed as DC circuits by being coupled with the decoupling capacitors.
Recently, however, active components such as ICs and LSI devices are rapidly developed in operating frequencies to become higher and higher. This indicates necessity of evaluation being performed on the power supply circuits with respect to their high frequency characteristics. Until now, however, no engineers actually propose techniques for evaluation of the high frequency characteristics of the power supply circuit by numerical analysis. As a result, the engineers related to technologies of printed-circuit boards suffer from problems, as follows:                (1) It is impossible to sufficiently guarantee quality in circuit operations of the printed-circuit boards.        (2) The printed-circuit boards radiate unwanted electromagnetic waves.        
Now, a description will be given in detail with respect to the problem (1). With respect to the decoupling capacitor, there are provided two kinds of impedance due to parasitic inductance, i.e., first impedance being caused by parasitic inductance of a decoupling capacitor itself and second impedance being caused by other parasitic inductance due to pads and via hole(s) for installing the decoupling capacitor on the board. When the operating frequency of the active component becomes higher, a sum of the first impedance and second impedance becomes large as compared with impedance corresponding to capacitance of the decoupling capacitor. For this reason, it becomes difficult to supply the active component with stable DC voltage. That is, it is hard to guarantee stable operations of the active component. Such difficulty will be described with reference to FIG. 28.
FIG. 28 is a sectional view showing a simplified construction of a decoupling capacitor, which is installed on a board (or substrate). Specifically, FIG. 28 shows a capacitor 220 being interconnected with an power layer 221 and a ground layer 222, which are formed inside of the substrate, by way of pads 223, 224 and a via hole 225. Herein, parasitic inductance of the capacitor 220 itself is approximately 1 nH if the capacitor is made as a chip component, while parasitic inductance of the pads 223, 224 and via hole 225 is approximately 1 nH in total. As the capacitator 220, it is possible to use a capacitor of a chip type of 0.1 μF, for example. In that case, however, the capacitor whose operating frequency is greater than 36 MHz does not act as a capacitive component any more but acts as an inductive component. For this reason, it is necessary to design the circuits such that the inductance is reduced as small as possible. If the circuits are designed not to reduce the inductance so much, variations of power voltage become large, so it may be impossible to guarantee normal circuit operations.
Next, a description will be given in detail with respect to the problem (2). When the operating frequency of the active component becomes high, lines inside of the power supply circuit do not act as simple equipotential lines but act as transmission lines. As a result, the power supply circuit as a whole acts as a resonance circuit with regard to the transmission lines. Due to resonance, the board installing the active component radiates strong electromagnetic waves. Such resonance will be described with reference to a concrete example of circuitry shown in FIG. 29.
FIG. 29 shows a circuit model that is designed more realistically to match with a board being actually manufactured. That is, the circuit model of FIG. 29 shows a power supply circuit (234) connected with multiple decoupling capacitors. In FIG. 29, an IC 205 is interconnected with a decoupling capacitor 230a and lines 231 that act as transmission lines. In addition, there are provided a decoupling capacitor 230b, which is being connected with another IC (not shown), and a DC power source 209 which is provided outside of the board. Both of the decoupling capacitors 230a, 230b have a similar configuration. That is, the decoupling capacitor (230a) includes capacitance 233 and parasitic inductance 232 in series. An overall area of the power supply circuit 234 is encompassed by a dashed line in FIG. 29. The power supply circuit 234 sometimes causes resonance.
FIG. 30 shows impedance characteristics with respect to frequencies. That is, a solid-line characteristic shows an example of the impedance characteristic based on impedance Zin of the power supply circuit 234 being observed from the IC 205, while a dotted-line characteristic shows an example of the impedance characteristic based on impedance Zc of the decoupling capacitor 230a. Herein, the impedance characteristic of Zin approximately matches with the impedance characteristic of Zc. But, the impedance characteristic of Zin greatly differs from the impedance characteristic of Zc at frequencies f01, f02. Differences between those characteristics are caused because of reasons as follows:
As shown by the dotted-line characteristic, the impedance Zc of the decoupling capacitor 230a functions as inductive reactance. In addition, the impedance of the power supply circuit 234, which is being connected to follow the decoupling capacitor 230a, functions as capacitive reactance. When both of the inductive reactance and capacitive reactance coincide with each other in magnitude, parallel resonance is being caused to occur.
When the resonance is caused to occur, the impedance Zin of the power supply circuit 234 being observed from the IC 205 becomes large. This causes variations of power voltage to become large. If resonance energy is accumulated in the power supply circuit, strong electromagnetic waves are radiated from the power supply circuit. In that case, it becomes hard to pass regulations specified by standards regarding EMI (i.e., electromagnetic interference) or radiation of unwanted electromagnetic waves.